1. Field
Embodiments disclosed herein relate generally to optical fibers and more particularly to ultra small core fibers.
2. Description of the Related Art
Ultra small core fibers (“USCFs”) have a variety of applications especially in devices that utilize optical nonlinearities. Ultra small core fibers have been used for supercontinuum generation, wavelength conversion, soliton-based pulse compression, and so forth.
U.S. Pat. No. 6,792,188 discloses a design where an inner layer of small holes is used to achieve tailored dispersion of a photonic crystal fiber. In this disclosure, a large number of air holes are used beyond the inner layer of small air holes. A significant drawback of this design is the need to use a large number of air holes beyond the inner layer of air holes to reduce confinement loss, especially for core diameters less than 2 μm.
In a paper by J. K. Ranka, et al., “Optical Properties of High-Delta Air Silica Microstructure Optical Fibers,” Optics Letters, vol. 25, pp 796-798, 2000, the authors disclose a fiber with a core diameter of 1.7 μm surrounded by a triangular arrangement of a large number of air holes with d/Λ≈0.9, where d is the diameter of an air hole and Λ is the center-to-center spacing of the air holes. The need for low confinement loss leads to the large air hole size (relative to the hole spacing) and the large number of air holes. The need for low confinement loss makes dispersion tailoring very difficult for a fixed core diameter.
U.S. Pat. No. 7,266,275 discloses a method of dispersion tailoring for a fiber incorporating a partially doped core to raise its refractive index. For small core diameters less than 2 μm, glass and air boundary plays a very significant part in confining light in the core. A refractive index change of a few percent over part of the core has very little impact on fiber dispersion.